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At least 289 records · Page 16

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NASA NTRS1985

Spatial and temporal patterns in pigment biomass in Gulf Stream warm-core ring 82B and its environs

A chronology of the horizontal and vertical distribution of phytoplankton pigment biomass provides a biological life history of Gulf Stream warm-core ring 82B. Development of ring 82B is followed through three distinct periods: a winter/early spring period of deep convective overturn with uniform and relatively high pigment concentrations to depths of 400 m, a late spring stratification period with subsurface (20-30 m) pigment maxima and relatively high ring center values compared to surrounding waters, and a Gulf Stream interaction period when ring characteristics are dominated by intrusions of low pigment concentration waters. While the ring maintains a unique identity throughout these periods of its life, prolonged deep vertical mixing as well as episodic interactions with its surrounding have a significant influence on pigment distributions within the ring. These interactions generally enhance pigment biomass compared to the water mass from which it is derived. A consistent feature of pigment distribution is the remarkable coherence between this measure of phytoplankton biomass and the corresponding physical hydrodynamic structure of the upper water column.

Smith, R. C.↗

NASA NTRS1985

Further explorations of cosmogonic shadow effects in the Saturnian rings

The mass distribution in the Saturnian ring system is compared with predictions from the cosmogonic theory of Alfven and Arrhenius (1975) in which matter in the rings was once a magnetized plasma, with gravitation balanced by centrifugal force and by the magnetic field. As the plasma is neutralized, the magnetic force disappears and the matter can be shown to fall in to a distance 2/3 of the original. This supports the cosmogonic shadow effect, also demonstrated for the astroidal belt and in the large scale structure of the Saturnian ring system. The relevance of the comogonic shadow effect for parts of the finer structures of the Saturnian ring system is investigated. It is shown that many structures of the present ring system can be understood as shadows and antishadows of cosmogonic origin. These appear in the form of double rings centered around a position a factor 0.64 (slightly 2/3) closer to Saturn than the causing feature.

Alfven, H.↗

NASA NTRS1985

The composition and structure of planetary rings

The properties of planetary ring systems are summarized herein; emphasis is given to the available evidence on their compositions and to their dynamical attributes. Somewhat contaminated water ice makes up the vast expanse of Saturn's rings. Modified methane ice may comprise Uranus' rings while silicates are the likely material of the Jovian ring. Saturn's rings form an elaborate system whose characteristics are still being documented and whose nature is being unravelled following the Voyager flybys. Uranus' nine narrow bands display an intriguing dynamical structure thought to be caused by unseen shephard satellites. Jupiter's ring system is a mere wisp, probably derived as ejecta off hidden parent bodies.

Burns, J. A.↗

NASA NTRS1986

What perturbs the gamma and delta rings of Uranus?

Since the discovery of the Uranian rings in 1977, it has been possible to determine the orbits of the rings with remarkable precision. It has been found that seven of the nine known rings follow well-behaved Keplerian ellipses. However, two adjacent rings, gamma and delta, deviate significantly from simple ellipses, with residuals of up to 10 km. The present report is concerned with a test of two possible causes of the observed perturbations in the case of the gamma and delta rings, taking into account shepherd satellites which are presumed to confine the sharp-edged narrow rings, and low-order Lindblad resonances.

French, R. G.↗

NASA NTRS1985

Saturn's rings - Particle size distributions for thin layer model

A model incorporating limited interaction between the incident energy and particles in the ring is considered which appears to be consistent with the multiple scattering process in Saturn's rings. The model allows for the small physical thickness of the rings and can be used to relate Voyager 1 observations of 3.6- and 13-cm wavelength microwave scatter from the rings to the ring particle size distribution function for particles with radii ranging from 0.001 to 20 m. This limited-scatter model yields solutions for particle size distribution functions for eight regions in the rings, which exhibit approximately inverse-cubic power-law behavior.

Zebker, H. A.↗

NASA NTRS1986

Voyager Saturnian ring measurements and the early history of the solar system

The mass distribution in the Saturnian ring system is investigated and compared with predictions from the plasma cosmogony. According to this theory, the matter in the rings has once been in the form of a magnetized plasma, in which the gravitation is balanced partly by the centrifugal force and partly by the electromagnetic forces. As the plasma is neutralized, the electromagnetic forces disappear and the matter can be shown to fall in to 2/3 of the original saturnocentric distance. This causes the so called 'cosmogonic shadow effect', which has been demonstrated earlier for the asteroidal belt and in the large scale structure of the Saturnian ring system. The relevance of the cosmogonic shadow effect is investigated for parts of the fine structures of the Saturnian ring system. It is shown that many structures of the present ring system can be understood as shadows and antishadows of cosmogonic origin. These appear in the form of double rings centered around a position a factor 0.64 (slightly less than 2/3) closer to Saturn than the causing feature. Voyager data agree with an accuracy better than 1 percent.

Alfven, H.↗

NASA NTRS1986

Electromagnetic angular momentum transport in Saturn's rings

It is shown here that submicrometer dust particles sporadically elevated above Saturn's ring are subject to electromagnetic forces which will reduce their angular momentum inside synchronous orbit and increase it outside. When the dust is reabsorbed by the ring the angular momentum of the ring is decreased (increased) inside (outside) of synchronous orbit. For the case of the spokes in Saturn's B-ring it is estimated that the timescale for transporting ring material due to this angular momentum coupling effect is comparable to the viscous transport time or even smaller. It is suggested that the minimum in the optical depth of the B-ring at synchronous orbit is due to this effect.

Goertz, C. K.↗

NASA NTRS1986

Slip ring experience in long duration space applications

Ball Aerospace experience with slip rings in space extends back to 1962. Over 40 multi-ring assemblies have been flown and continuous operating lifetimes greater than 8 years at up to 60 rpm have been demonstrated. Slip rings provide multi-channel transfer of electrical power and signals in assemblies that are small in size and weight, and low in cost. By use of multiple brushes and sufficient copper within the assembly, power transfer efficiency better than 99.95 percent for high voltage circuits can be achieved. A low slip ring failure rate based on actual space operation totalling billions of ring revolutions has been established. Well qualified suppliers who have been making slip rings for space use for over 25 years are available. It is hoped that the suspected problem in SEASAT will not be allowed to prejudice space system designer against these very useful mechanisms.

Phinney, Damon D.↗

NASA NTRS1986

Structure and evolution of Saturn's rings

It is noted that many Voyager observations on the structure and evolution of Saturn's rings remain largely unexplained. The variation of ring thickness and particle size with composition may be partly explained by the input of density waves in the 'heating' of such rings as the outer A ring; in addition, the particles appear to resemble Weidenschilling et al.'s (1984) 'ephemeral bodies' rather than chunks of ice. It is suggested that many current difficulties may be resolved by positing that at least ring A is young, having been created by the destruction of one of the ring moons.

Esposito, L. W.↗

NASA NTRS1987

Ring current and radiation belts

Studies performed during 1983-1986 on the ring current, the injection boundary model, and the radiation belts are discussed. The results of these studies yielded the first observations on the composition and charge state of the ring current throughout the ring-current energy range, and strong observational support for an injection-boundary model accounting for the origins of radiation-belt particles, the ring current, and substorm particles observed at R less than about 7 earth radii. In addition, the results have demonstrated that the detection of energetic neutral atoms generated by charge-exchange interactions between the ring current and the hydrogen geocorona can provide global images of the earth's ring current and its spatial and temporal evolution.

Williams, D. J.↗

NASA NTRS1987

Collision mechanics and the structure of planetary ring edges

The present numerical simulation of collisional evolution, in the case of a hypothetical ring whose parameters are modeled after those of Saturn's rings, gives attention to changes in radial structure near the ring edges and notes that when random motion is in equilibrium, the rings tend to spread in order to conserve angular momentum while energy is dissipated in collisions. As long as random motion is damped, ring edges may contract rather than spread, producing a concentration of material at the ring edges. For isotropic scattering, damping dominates for a coefficient of restitution of velocity value of up to 0.83.

Spaute, Dominique↗

NASA NTRS1987

The earth's ring current - Present situation and future thrusts

Particle distributions, currents, and the ring current situation prior to the August 1984 launch of the AMPTE Charge Composition Explorer (CCE) are discussed. CCE results which demonstrate the capability of these new measurements to pursue questions of ring current sources, energization, and transport are presented. Consideration is given to various ring current generation mechanisms which have been discussed in the literature, and a two-step generation process which to a certain extent unifies the previous mechanisms is presented. The first in-situ global observations of ring current decay as obtained through the detection of energetic neutral atoms generated by charge exchange interactions between the ring current and hydrogen geocorona are discussed, as well as the possibility of using the detection of energetic neutral atoms to obtain global images of the earth's ring current.

Williams, D. J.↗

NASA NTRS1987

The rings of Uranus

The rings of Uranus are not all circular; nor do they all lie in the plane of Uranus' equator. The epsilon ring of Uranus, which is the largest and outermost one, ranges in width from 20 to 96 km in proportion to the distance of the ring material from Uranus. Typical rings can be modeled by assuming that its edges are defined by a combination of smooth elliptical orbits and random 'wiggles'. A transition from a few large moons in outer regions to many small moons just outside ring-filled inner regions is a feature noted to be held in common by the ring and moon systems of Saturn, Uranus, and Neptune.

Cuzzi, Jeffrey N.↗

NASA NTRS1988

Uranian ring orbits from earth-based and Voyager occultation observations

A novel kinematical model of the Uranian rings has been derived from Voyager occultation timings and earth-based observations of the occultations of U23, U25, and U28, to yield an improved orbit solution that is accurate to about 1 km in the ring plane; this allows a critical comparison to be conducted between ring locations and suspected resonances. The gamma ring is noted to have a peculiar orbit that combines a precessing elliptical ring with a radially-oscillating normal mode. The kinematical model confirms the suspected association of resonances between the 1986U7 and 1986U8 satellites with some of the rings.

French, Richard G.↗

NASA NTRS1988

The Titan-1:0 nodal bending wave in Saturn's Ring C

The most prominent oscillatory feature observed in the Voyager 1 radio occultation of Saturn's rings is identified as a one-armed spiral bending wave excited by Titan's-1:0 nodal inner vertical resonance. Ring particles in a bending wave move in coherently inclined orbits, warping the local mean plane of the rings. The Titan-1:0 wave is the only known bending wave that propagates outward, away from Saturn, and the only spiral wave yet observed in which the wave pattern rotates opposite to the orbital direction of the ring particles. It is also the first bending wave identified in ring C. Modeling the observed feature with existing bending wave theory gives a surface mass density of about 0.4 g/sq cm outside the wave region and a local ring thickness of less than about 5 meters, and suggests that surface mass density is not constant in the wave region.

Rosen, Paul A.↗

NASA NTRS1989

O-ring sealing verification for the space shuttle redesign solid rocket motor

As a part of the redesign of the Space Shuttle Solid Rocket Motor, the field and nozzle-to-case joints were redesigned to minimize the dynamic flexure caused by internal motor pressurization during ignition. The O-ring seals and glands for the joints were designed to accommodate both structural deflections and to promote pressure assistance. A test program was conducted to determine if a fluorocarbon elastomeric O-ring could meet this criteria in the redesigned gland. Resiliency tests were used to investigate the O-ring response to gap motion while static seal tests were used to verify design criteria of pressure assistance for sealing. All tests were conducted in face seal fixtures mounted in servo-hydraulic test machines. The resiliency of the O-ring was found to be extremely sensitive to the effects of temperature. The External Tank/Solid Rocket Booster attach strut loads had a negligible affect on the ability of the O-ring to track the simulated SRB field joint deflection. In the static pressure-assisted seal tests, as long as physical contact was maintained between the O-ring and the gland sealing surface, pressure assistance induced instantaneous sealing.

Lach, Cynthia L.↗

NASA NTRS1987

The role of resonances in planetary rings

The new observations of planetary rings, including those acquired during the encounters of Voyager with Jupiter, Saturn, and Uranus, and the discovery of incomplete rings around Neptune, reveal the great importance of resonances in determining the dynamics and the shape of planetary rings. Several types of resonances play a part in planetary rings. Current questions of interest are related to the nonlinear theory of density waves, the confinement of the Uranian rings, and the arcs of rings around Neptune.

Borderies, N.↗

NASA NTRS1989

Neptune may have polar rings

Perturbations from Neptune's highly inclined satellite Triton can maintain rings passing nearly over Neptune's poles. These hypothetical polar rings are nearly perpendicular to Triton's orbit as well, and lie within several degrees of the plane of Voyager II's trajectory through the Neptunian system. Polar rings can coexist with equatorial rings at different radii. A randomly oriented torus of debris around Neptune has a probability of several percent to settle into a polar ring. Voyager II stands a significant chance of encountering a polar ring.

Dobrovolskis, Anthony R.↗